The present invention relates generally to optical filters, and more particularly to improved high transmission comb-type optical filters contained in waveguiding media within which or onto which multiply periodic and/or stepwise constant modulation is imposed in order to effect strong reflections at a multiplicity of selected narrow optical bands occurring within a larger selected band.
High transmission comb (HTC) filters are distinguished from Fabry-Perot (F-P) type filters by their design and performance characteristics. To quantify uniqueness, the reflectance of individual filter elements is specified to be less than about 70 percent which is less than one would expect to find for the mirrors of a simple F-P resonant structure or any among a series of optically coupled F-P type resonant structures.
A summary of prior work on HTC filters may be found in Land et al, U.S. Pat. Nos. 5,170,290 and 5,225,930, and "High Transmission Comb-Like Optical Filters," J Optical Society of America A, pp 611-622 (1995). Land et al '290 teaches thin film filters where the modulation is stepwise constant with one step, of length d.sub.1 being much shorter than the other d.sub.2. This type filter has a periodic length L.sub.R =d.sub.1 +d.sub.2 which is substantially longer than the wavelength range to be filtered, so that the filter operates in high order. Land et al '930 teaches multiply periodic modulation (mpm) with form that can be interpreted to include the form of the structure described in the '290 patent.
The coupled mode perturbation theory as taught by Yariv et al, Optical Waves in Crystals (John Wiley & Sons, NY (1984)) was extended by Land et al in order to conveniently design and characterize multiply periodic thin film filters. The theory is applicable to waveguides which confine wave propagation within 1 or 2 major dimensions and to unbounded propagation. (The term waveguide applies to any wave guiding structure that limits the region of wave propagation. Mpm of a waveguide may include the core, the relatively high index region, where waves propagate, and any region between or adjacent individual waveguides which support only evanescent waves.)
The invention provides an mpm filter structure having a comb-like reflection spectrum wherein the envelope connecting significant peak reflectances spans an optical band of interest.
The prior art teach HTC filters that control light transmission through index modulated thin films prepared on substrates and are structured so that the modulated length L is small compared to the lateral extent of the film. The HTC filter structure of the invention includes waveguiding media in which the modulated length L is large with respect to at least one waveguide dimension.
Singly periodic modulation comprising short and long segments with a periods substantially longer that the wavelength range of interest also produces a comb spectrum with narrow reflection peaks and moderate to high total transmission, but is not as wavelength selective, nor efficient in providing reflection per unit length in materials having limited modulation amplitude, as mpm.
Mpm provides for cascaded multiplexing/demultiplexing (m/d) of interlaced frequencies, whereas other Bragg type modulation typically provides serial branching of frequencies. Mpm may provide better frequency discrimination than a Mach Zender structure or arrayed waveguide gratings because of the difference in spectral transmission/reflection and branching characteristics. Mpm may be more economically produced, and coherent mpm can be provided in silica optical fiber directly and in other holographic media by crossed ultraviolet beams without subsequent curing.
Mpm may beneficially replace or augment other means for providing feedback to waveguide lasers and may permit tunable or switchable multiline distributed feedback lasers. Mpm be used as novel dielectric mirrors for resonant waveguide cavities. The mirrors might be identical and with proper spacing could allow transmission of a narrow line within some reflection peaks of a set of reflection comb peaks. Another version may have an mpm reflector pair with different free spectral ranges bounding a section of unmodulated waveguide with the mpm filter comb peaks somewhat offset so that by tuning one reflector set, a single resonance supports transmission of a single narrow line. This configuration might be applied to development of switchable and tunable waveguide lasers. A single narrow line can be separated from other transmission by an auxiliary filter.
Typically, thin film filters and waveguides present different advantages and limitations with respect to employing mpm. The thin films provide relatively large optical index modulation, and the average index may be independent of the modulation amplitude. In the case of modulated optical fibers, for example, the variation in index is typically limited to index modulation levels near or below 0.1%, and the average index changes with dose and the modulation characteristic may change with dose. Thin film filter modulation thicknesses are limited because of loss of physical integrity and by slow deposition rates. Coherence may be reduced both laterally and through the thickness. The coherence length of modulation in optical fibers can be much greater, perhaps 1000 times greater than in thin films. Coherent modulation and consistent performance are achieved by controlling waveguide uniformity and ambient temperature and by designing to reduce and/or compensate the effects of thermal expansion and thermally induced index change. However, some applications may utilize a shift in a comb spectrum for control or measurement.
One motivation for using mpm in optical filters is to provide behavior opposite to that of an F-P type filter, that is, a comb-like reflection which contrasts with the comb-like transmission of an F-P filter. Another motivation is to provide alternatives to simple serial or simple parallel Bragg modulation to provide a comb-like reflection and transmission spectrum in cases where a regular comb peak spacing (free spectral range) will serve as well as an arbitrary peak spacing (Othonos et al, "Superimposed multiple Bragg gratings," Electronics Lett 30, No 23, pp 1972-4 (1994)). Although it cannot be arbitrary and cannot be strictly uniform in spacing by wavenumber, the line spectrum of an mpm HTC waveguide filter can be useful.
Mpm may be advantageous relative to serial or parallel Bragg modulation when a large number of narrow spectral lines are desired within a given spectral range, most particularly if the refractive index excursion of a medium to be modulated is limited (Othonos et al, supra, and Chen, "Polymer-based photonic integrated circuits," Optics and Laser Tech, 25(6), pp 347-365 (1993)). This advantage can be understood by comparing the spectrum generated by serial modulation using four Bragg elements in tandem and the spectrum when the four elements are divided into smaller sections having the same modulation amplitude and arranged as an mpm (Land et al, supra). The latter arrangement provides an increased number of reflection peaks with smaller half widths, while providing about the same total transmission and total reflection. However, if the serial modulation is replaced by parallel modulation throughout the filter length with the index modulation amplitude of each component taken as 1/4 that of the other arrangements to avoid saturation effects, the number and amplitude of reflection peaks are equal to those of the serial case. However the peak widths for the parallel case will be about the same as the corresponding peaks provided by the mpm. These differences are illustrated partially in FIGS. 6 and 7 of Land et al, JOSA, supra. The differences are rooted in the fact that spectral resolution of a filter generally increases with modulation length and the fact that the total reflection relates to the average modulation amplitude.
It is therefore a principal object of the invention to provide an improved optical filter.
It is another object of the invention to provide an optical filter for improving optical waveguide devices and related systems.
It is another object of the invention to provide a high transmission comb-type optical filter having multiply periodic refractive index modulation.
It is another object of the invention to provide an mpm waveguide.
It is another object of the invention to provide a novel resonant F-P optical cavity structure.
It is a further object of the invention to provide mpm waveguide reflectors which include m/d of optical waves in optical fibers and waveguides, multiline distributed feedback lasers, multiline Raman lasers, fiber and waveguide sensors and sensor systems.
These and other objects of the invention will become apparent as a detailed description of various embodiments thereof proceeds.